Total hip arthroplasty (THA) designs have been continuously altered to improve fixation and range of motion, and thereby avoid problems of Several studies have investigated a variety of prostheses with varied neck-shaft angles, neck geometries, head diameters, and socket details. owever, in meeting these criteria, prosthetic design parameters must conform to certain anatomical and geometrical constraints. Two major parameters that the surgeon can select to optimize for the patient's anatomical variation are head size and neck length. To better understand the influences and trade-offs of head diameter and neck length with respect to THA motion, this study investigated their effects and compared prosthetic range of motion with that of the natural hip. Also noted, were the types of contact limiting range of motion, i.e., bony or prosthetic.A cadaveric pelvis was mounted in a three-dimensional protractor as previously described.* The pelvis provided the same geometric and bony constraints found in vivo, while providing for accurate motion measurements and contact observations. Range of motion was assessed for the natural hip using two types of tests, each conducted at 0" and 30" abduction as follows:( 1 ) maximum internal and external rotation were measured at 10" flexion increments, increasing from 50" extension to 120" flexion; and(2) maximum flexion and extension were measured at 10" rotation increments from 120" external to 120" internal rotation.A S T H total hip prosthesis was selected to assess the influence of head diameter and neck length on range of motion. The neck of the prosthesis was machined to accept adaptors for necklength variations and interchangeable femoral heads (Fig. 1 ). The polyethylene acetabular component was machined to accept the varied head sizes such that the center of rotation and the contact geometry between the femoral neck and acetabular socket were kept constant for all configurations. The femoral stem was implanted at 5" anteversion to the femoral shaft and 0" varus, and the acetabular cup was mounted with a lateral opening of 45" and at 20" anteversion ( Fig. 2). Modeling clay was used to stabilize the components until correct positioning was confirmed radiographically. The components were then locked in place with a collar of acrylic bone cement. Range of motion and type of contact limiting motion (bony or prosthetic), were tested as above for a 22 mm head diameter with a 32 mm neck length, a 32 mm head diameter with a 32 mm $00.90 0 J. B. Lippincott Co. 284
In view of published comments that the wear resistance of Ti-6Al-4V alloy bearing against ultrahigh molecular weight polyethylene (UHMWPE) was not adequate for total joint replacements, the literature was reviewed and compared with ongoing hip simulator studies. Two types of laboratory tests were contrasted: those run under “clean” conditions and those run with intentional contamination by acrylic cement particles. Wear-screening devices as well as hip simulators were involved in both types of studies, and both produced a dichotomy of viewpoints on the suitability of the Ti-6Al-4V/UHMWPE combination. However, the two most recent studies with hip simulators indicated that the Ti-6Al-4V-UHMWPE combination performed acceptably well. The published data from a major series of Ti-6Al-4V hip replacements have established excellent clinical and radiographic results. In seven surgical revisitations, the Ti-6Al-4V/UHMWPE combinations showed no sign of adverse wear phenomena. Thus, the clinical observations are supportive of the results of the two most recent hip simulator studies. It may well be that the laboratory models produced a variety of severe test conditions that compromised the performance of the Ti-6Al-4V and resulted in contradictory data. The hip simulator studies are being continued to establish the limits of the laboratory models and their validity with respect to the clinical situation.
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